Embodiments relate to cementing operations and, more particularly, in certain embodiments, to methods and compositions that utilize high-alumina refractory aluminosilicate pozzolans in well cementing.
In cementing operations, such as well construction and remedial cementing, cement compositions are commonly utilized. Cement compositions may be used in primary cementing operations whereby pipe strings, such as casing and liners, are cemented in wellbores. In a typical primary cementing operation, a cement composition may be pumped into an annulus between the exterior surface of the pipe string disposed therein and the walls of the wellbore (or a larger conduit in the wellbore). The cement composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable material (i.e., a cement sheath) that may support and position the pipe string in the wellbore and may bond the exterior surface of the pipe string to the wellbore walls (or the larger conduit). Among other things, the cement sheath surrounding the pipe string should function to prevent the migration of fluids in the annulus, as well as protect the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, such as in squeeze cementing for sealing voids in a pipe string, cement sheath, gravel pack, subterranean formation, and the like. Cement compositions may also be used in surface applications, for example, construction cementing.
In order for the cementing operation to be successful, the cement composition should satisfy a number of specific requirements, including rheology, fluid loss, thickening time, and strength development. When used in high temperature wells (e.g., >250° F.), conventional cement compositions comprising Portland cement are known to lose strength after continued exposure to high temperature, a phenomenon commonly referred to as “strength retrogression.” It is believed that strength retrogression of Portland cement occurs because the normal bonding phase (calcium hydroxide) is transformed to alpha-dicalcium silicate at elevated temperatures. Strength retrogression may be combated by the addition of silicon dioxide materials to the cement composition, including silica flour or silica sand. While silicon dioxide materials have been used successfully to counteract strength retrogression, it is desired to develop additional cement compositions that can be used at elevated temperatures.